a) Field of the Invention
The present invention relates to a reflection type display, and more particularly to a reflection type display with a front light.
b) Description of the Related Art
A liquid crystal display will now be described as an example of a display using electro-optical material which changes optical characteristics in accordance with electric field, however, the present invention is not limited to that.
Reflection type liquid crystal displays have been known as one type of the liquid crystal displays. A reflection type liquid crystal display is expected as a suitable display for an information terminal or the like because of low power consumption.
However, images displayed on the reflection type liquid crystal display are invisible in dark environment because the display has no light source. To overcome this disadvantage, a reflection type liquid crystal display having a front light on its viewer's side has been proposed. For example, such the reflection type liquid crystal display comprises a wedge-shaped light guide plate having a light source on its side. The light guide plate is placed on the viewer's side of the display panel. The wedge-shaped light guide plate has a planar bottom surface and a prismatic top surface comprising planar surfaces and slanted (oblique) surfaces which are arranged alternately.
The lights are introduced into the light guide plate from the light source which is placed on the side of the light guide plate. The slanted surfaces of the light guide plate reflect the introduced lights toward the liquid crystal display. A reflection plate of the liquid crystal display reflects the lights toward the viewer's eyes through the light guide plate. That is, the wedge-shaped light guide plate works as a reflector and a light transmitter.
To work as an ideal light transmitter, the planar surfaces of the prismatic top surface of the light guide plate must be almost parallel to the bottom surface. If each of the planar surfaces of the prismatic top surface is formed at a large angle with the bottom surface, the reflected lights from the liquid crystal display are refracted by the light guide plate because it works as a prism. Such the refraction influences the display quality of the liquid crystal display.
In order to reflect lateral lights in the light guide plate so as to incident upon the liquid crystal display at almost right angle, each of the slanted surfaces is required to be formed at an angle of approximately 45 degrees with the bottom surface.
The light guide plate having such the slanted surfaces and planar surfaces arranged alternately will become tapered, that is, wedge shaped. For example, the height around the light incident side of such the wedge-shaped light guide plate differs from the height around the light outgoing side, and the difference is approximately 2 to 3 mm. Such the wedge-shaped light guide plate may deteriorate the display quality of the liquid crystal display. In a case where a touch panel is disposed on the liquid crystal display, for example, a wedge-shaped gap appears between the liquid crystal display and the touch panel because of the wedge shaped light guide. This gap causes parallax.
As stated above, the reflection type liquid crystal display with the front light can be driven with low power consumption because the light source is activated only when the user needs it. However, employing the front light also brings several disadvantageous matters.